1. Field of the Invention
The present invention relates to an electric power converting device for supplying electric power from a commercial power source to a load and, in particular, to an electric power source device for supplying electric power to a lamp such as a discharge lamp, including a fluorescent lamp, or a high intensity discharge lamp including a metal halide lamp and a high pressure sodium lamp.
2. Description of the Prior Art
In the prior art an electric power converting device utilizing switching elements, in order to reduce the harmonic distortion of the input current and also to increase the input power factor, it is known to use at the front stage of a main converting circuit a circuit (hereinafter referred to as a front converting circuit) that is designed to full-wave rectify the commercial alternating current (AC) source and then to shapeing the waveform of the input current into a waveform substantially proportional to the commercial AC power source as to generate a direct current voltage. In this type of the power converting device, using the direct current voltage from the front converting circuit, the main converting circuit provides the load with the desired electric power. By way of example, in a ballast for supplying a high frequency AC power to a fluorescent tube, the main converting circuit is comprised of a booster type chopper circuit and an inverter circuit.
However, with this type of electric power converting device, since the number of component parts added to the front converting circuit (the booster type chopper circuit) for reducing the input current harmonic is relatively large the device itself tends to be bulky in size and high in cost.
In view of the foregoing, various circuits have hitherto been suggested which employ a reduced number of component parts and are lower in cost as compared with those employed in the circuit utilizing a booster type chopper circuit and an inverter circuit, some of which will now be discussed.